1. Technical Field
This invention is directed generally to devices and methods for facilitating bone growth, and, in particular, to bone implantable devices and implantation methodologies that augment beneficial bone growth or repair while limiting bone growth in undesirable directions.
2. Background
In orthopedic and neurological surgical procedures it is often important to facilitate the growth or fusion of bony structures. This may entail growth “bone-to-bone” or, depending on the nature of the procedure, bone to device.
Chronic back problems, for example, cause pain and disability for a large segment of the population. In many cases, such problems are attributable to relative movement between vertebrae in the spine. Spinal surgery includes procedures to stabilize adjacent vertebrae. Common stabilization methods often involve fusing adjacent vertebrae together.
Fusion techniques include removing disc material that separates the vertebrae and impacting bone into the disc area. The impacted bone fuses with the bone material of the two adjacent vertebrae to thereby fuse the vertebrae together.
In a further advance in the art, spinal implants have been developed to increase the probability of a successful fusion. Such devices generally comprise a hollow cage into which bone growth inducing substances, such as bone chips or bone slurry, may be placed. The cage is inserted, either by anterior or posterior approach, into the intervertebral disc space. The cage wall has holes extending radially therethrough, typically throughout the entire cage surface. Bone growth extends into the device through the radial apertures, facilitating arthrodesis between the adjacent vertebral bone structures and allowing for the decompression of neural elements.
With the continued development of techniques for achieving spinal fusion through the use of spine fusion cages, new materials have been developed to augment the fusion process. Traditionally, the patient's own bone, or cadaver bone, was used in the cage to promote bony fusion. More recently, powerful new biologic materials have been discovered that greatly accelerate the fusion process, in some cases eliminating the need for donor bone.
However, with the utilization of the newer biologic materials there has arisen a significant problem. When bone growth inducing agents, such as bone morphogenic proteins (“BMP”), are used in cages of existing design there is risk of inducing the overgrowth of bone around and into sensitive neural tissues. This is especially the case when a posterior approach is utilized to implant a spinal fusion cage, as bony overgrowth toward the central canal or neural foramen may impinge on spinal nerve roots causing neurological damage. A recent study on posterior lumbar interbody fusion procedures using rhBMP-2 reported that 58% of patients experienced greater than expected bone formation dorsal to the fusion cage. In 30% of the cases, the bony overgrowth compromised the central canal, the neural foramen, or both. This study is confirmatory to observations first made by the present inventor in early 1999.
Typically, the bone growth agent is in liquid form and is applied to an absorbent carrier material, such as a piece of bovine collagen. The doped carrier material is placed with forceps into the interbody space, usually into an open end of the fusion cage after the cage has been implanted, but sometimes prior to cage implantation. During placement carrier material may inadvertently wipe across body areas, including internal bony structures, where bone growth is not desired. In addition, as the carrier material is pressed into place agent may squeeze out and flow into adjacent areas. Exacerbating the problem, current protocols do not encourage the use of suction, irrigation and hemostatic agents when bone growth agent is utilized. Conventional cage design also allows for the leakage of agent into undesirable areas after implantation through ill-placed apertures in the cage body, in the cage end caps, or otherwise. Because of the powerful stimulatory effects of bone growth agents, uncontrolled application of these substances may lead to serious complications, including severe inflammation, debilitating neural impingement, and other potential complications.
Thus, there is a need to better control the bone growth process when using a bone implantable device, especially in circumstances where powerful bone growth inducing agents are used in conjunction therewith.
In satisfying this need, there is also an opportunity to extend the application of bone growth agent based bony fusion to all types of bone implantable devices to better achieve union of bone-to-bone or bone-to-device, as the case may be.